The information Bill, Mark, and Dan provided you is correct. I just replaced the ball valve in my forward head with the Groco part number IBV-1250BSPP. Because it is a straight thread you will need to use a thread sealant to both seal the threads and to fix the ball valve in a position where you can access and turn the handle. I used Loctite 592 Thread Sealant which has worked out well so far.
SM #440 Cara
Deale, MD USA